This invention is concerned with eliminating lock-in problems in ring laser gyroscopes.
A mechanical gyroscope utilizes the inertia of a spinning mass to provide a reference direction useful in various applications, such as the navigation of an airplane or a spacecraft. The moving parts required in a mechanical gyroscope, however, introduce some undesirable attributes, such as high drift rates resulting from friction, into the device. The ring laser gyroscope was developed to avoid some of these difficulties.
The ring laser gyroscope maintains a constant frame of reference by circulating massless light waves in a closed path. A typical ring laser gyroscope, for example, consists of a resonant cavity defined by three or four corner mirrors. A gas laser generates a monochromatic light beam which is split into two beams. These beams are made to propagate in clockwise and counterclockwise directions in the cavity. If the gyroscope is rotated about an axis which has a component normal to the plane of the optical path, the frequency of one of the beams will be increased, while the frequency of the other beam will decrease, because of the Doppler effect. The beams can then be extracted from the cavity and combined to produce a beat frequency which can be related to the magnitude and direction of the rotation.
In an inertial frame, the optical path lengths for the clockwise and counterclockwise beams of a conventional ring laser cavity are exactly the same. Thus, when the ring gyroscope lases, the two beams will exhibit exactly the same frequency. If the gyroscope is rotating, the effective optical path lengths for the clockwise and the counterclockwise beams are different. In practice, the two beams tend to oscillate at the same frequency for small rotation rates. This is known as the "lock-in" problem and is due to the coupling of the two beams which results from backscattering of the laser beams by the mirrors in the beam path. Backscattering causes a small amount of light from each laser beam to be transferred to the oppositely traveling wave. At slow rates of rotation, this coupling causes the frequencies of the two beams to lock together at a single frequency, thereby preventing the measurement of slow rotation rates.
The lock-in problem is conventionally avoided by mechanically vibrating the ring cavity or using the magneto-optic effect to cause the two beams to oscillate at different frequencies in an inertial frame. If this frequency difference is made large enough, lock-in will not occur. The art of ring laser gyroscopes would be considerably advanced, however, if the oscillation frequencies could be split with a simpler, more compact, and more reliable technique.
Another technique for reducing the lock-in frequency requires the addition of a wavefront conjugating element to the ring cavity (Diels, U.S. Pat. No. 4,525,843; Diels, et al., Influence of wave-front-conjugated coupling on the operation of a laser gyro, Optics Letters, Volume 6, Page 219 (1981)). The intracavity conjugating element causes a fraction of the energy in each beam to be phase shifted and added to the oppositely-directed beam, thereby introducing a coupling between the counter-rotating beams similar to the coupling caused by backscattering. The phase shifts introduced by the wave front conjugation are cumulative with the phase shifts due to rotation, so that the frequency at which lock-in of the gyro occurs is reduced. Because this wave front conjugation process is symmetrical, however, the phase shift introduced in the clockwise wave by the conjugated coupling is equal in magnitude to the phase shift introduced in the counterclockwise wave and the two frequencies of oscillation remain identical when there is no rotation or rotation at a rate below the threshold value. Consequently, lock-in will still occur at frequencies below the lowered threshold frequency and this technique is thus only a partial solution because it does not completely eliminate the lock-in problem.